Laser beam scanning systems are widely used as for example in the exposure of printing plates, inspection of printed circuit boards, and the like. They fall into two general categories;
(1) those that use pre-deflection optics, i.e. the laser beam passes through a focusing lens before it is deflected as by a rotating polygon, oscillating mirror, or the like, and PA1 (2) those that use post-deflection optics, i.e. the laser beam passes through focusing optics after it is deflected.
In the pre-deflection optic case, a very simple inexpensive lens is adequate to produce a diffraction-limited focused spot. However, with such systems the locus of the focused spot is not a straight line but is, in general, a curved line. If, for example, the deflector is an oscillating mirror whose axis of rotation is in the plane of the mirror, the locus of the focused spot is an arc of a circle. Generally, the desired path of the focused spot is a straight line in which case the laser beam will be out of focus on the straight line except at one or two points where the arc is tangent to or intersects the line.
In some post-deflection systems a complex, expensive lens is employed in an attempt to maintain the laser beam in focus on a straight line. Because the angular field-of-view of such a lens must be large, lens design compromises often are made whereby not only is the focused spot degraded from perfection (diffraction limited) but also the locus of the best focus deviates from a straight line.
In some applications, such as in the inspection of printed circuit boards, it is highly desirable to cause the beam to be incident perpendicular to the scanned surface. This is not possible in prior art predeflection optics systems. A post-deflection configuration is known using a telecentric lens in which the scanning beam is normal to the scanned surface. However, the telecentric lens must be at least as large as the scan line is long and therefore telecentric systems are prohibitably expensive except for systems that require only very short scan lines.